1. Field of the Invention
The present invention relates to an image forming device in which a photoconductive drum is charged by a corona discharge method. In particular, the present invention relates to an image forming device which optimizes the photoconductive drum and a peripheral system of the photoconductive drum at a start of a print job by carrying out a pre-rotation process.
2. Description of the Related Art
Many copy machines and facsimile machines or the like function as an image forming device which forms an image on printing paper according to image data scanned from an original document or image data received by facsimile. In the image forming device, light is irradiated on a uniformly charged photoconductive drum according to the image data to be printed, and an electrostatic latent image is formed. Toner is supplied onto the electrostatic latent image, and the electrostatic latent image is formed into a toner image. Then, the toner image is transferred onto the printing paper. As one charging method of the photoconductive drum, a corona discharge method has been conventionally used. The corona discharge method is a non-contact charging method. Under the corona discharge method, a high voltage is applied to a tungsten corona wire or the like. Accordingly, a corona discharge generates between the corona wire and the photoconductive drum. When the corona is being discharged, ions generated as a result of ionization of the surrounding air are guided towards the photoconductive drum. As a result, the surface of the photoconductive drum is charged.
In the image forming device using the corona discharge method, when the corona is being discharged, discharge product gases such as NOx and O3 are generated. There exists a problem that a normal image forming process is inhibited by the discharge product gases. For example, NOx combines with moisture in the air to form HNO2. When HNO2 adheres onto the photoconductive drum, HNO2 melts the surface of the photoconductive drum to make the surface uneven. As a result, the charging of the surface of the photoconductive drum is inhibited. Additionally, O3 prevents a movement of the charge on the photoconductive drum. As a result, a formation of an electrostatic latent image is blocked. Therefore, a fan or the like is arranged in the image forming device to discharge the corona product gases to the outside of the image forming device. However, when the discharge product gases are not completely discharged and remain inside the image forming device, the discharge product gases pollute the surface of the photoconductive drum. Accordingly, during a pre-rotation process carried out at a start of a print job, the polluted surface of the photoconductive drum is required to be refreshed. The pre-rotation process is a process for adjusting the photoconductive drum and the peripheral system of the photoconductive drum into an optimum state by securing a given period of time at the start of the print job in order to enable a normal image forming process. During the pre-rotation process, for example, since a cleaning blade makes contact with the photoconductive drum, the polluted surface of the photoconductive drum is abraded and a new surface is exposed.
The period of time required for the pre-rotation process changes according to a surrounding environmental condition. Therefore, a conventionally proposed method provides a temperature sensor for measuring an outside air temperature and optimizes a pre-rotation period according to this measurement result.
However, in the above-described conventional image forming device, no consideration is made regarding an aspect that a level of pollution of the photoconductive drum changes according to a period from a completion of a previous print job until a start of a next print job. That is, when the image forming device is left for a long period of time without a print job being executed, for example, from an end of a week until a beginning of the next week, the level of the pollution on the surface of the photoconductive drum caused by the discharge product gases is high. Meanwhile, when a next print job is started immediately after a completion of a previous print job, the level of the pollution on the surface of the photoconductive drum is low. However, in the conventional image forming device, an execution period of the pre-rotation process is constant regardless of a job interval. Therefore, when the image forming device is left for a long period of time and the level of the pollution of the photoconductive drum is high, a normal image forming process cannot be carried out due to insufficient cleaning. Meanwhile, when the image forming device is left for a short period of time and the level of the pollution of the photoconductive drum is low, the pre-rotation process is carried out over an unnecessarily long period of time. As a result, the start of the next print job is delayed.